In a Downlink (DL) Multi-User Multiple-Input Multiple-Output (MU-MIMO) system, an access point (AP) with multiple antennas can simultaneously transmit data to multiple wireless terminal stations (STAs) by spatial multiplexing via different spatial paths. Due to spatial multiplexing, MU-MIMO offers the advantages of high peak throughput of data transmission. Additionally, MU-MIMO can reduce the number of channel accesses and thereby reduce overhead time associated with using short packets. Therefore, significant aggregation gain can be achieved.
Digital beamforming has been widely adopted by wireless communication systems, e.g., systems compliant with IEEE 802.11ac (5 GHz), which typically use omni-antennas for non-Line-Of-Sight (NLOS) MU-MIMO communication. Due to the relatively long wavelength used for signal transmission, propagation directionality is typically poor. Digital beamforming for MU-MIMO, based on the null space operation, for each spatial stream transmission all the receiving wireless stations except for the target receiving wireless station are in the nulls, is susceptible to fast channel aging. To combat channel aging, frequent sounding and feedback processes are used (for instance greater than 50 times/sec at 5 GHz), but these processes inevitably contribute to substantial overhead time. Further, the data traffic needed for reporting Channel State Information (CSI) increases with the number of antennas and the number of STAs used in a system. This inhibits the system's ability to scale up a user group.
For higher frequency systems (e.g., 60 GHz), the wavelength is very small compared to the feature sizes of objects in the propagation environment. Therefore, signal propagation at such a high frequency is less affected by the propagation environment and therefore can maintain high directionality in discrete spatial paths, making these systems suitable for analog beamforming using directional phased-array antennas. Null space type of digital beamforming suffers much fast channel aging at the short wavelength (aging rate proportional to the frequency), and is generally only used as an enhancement after analog beamforming is accomplished.
MU-MIMO, unlike SU-MIMO, can transmit multiple spatial streams via LOS transmission to multiple users. LOS MU-MIMO transmission can potentially provide even higher throughput and longer beam range because of mitigated path and reflection loss as normally associated with NLOS-MIMO transmission.
Hybrid beamforming combines analog beamforming and digital beamforming processing, and has been acknowledged to be a more reliable approach for MU-MIMO millimeter-wave (mmWave) beamforming communication than digital beamforming.